1. Field of the Invention
The present invention relates to a control apparatus for a hydraulically-operated vehicular transmission having hydraulic engaging elements, mainly for use in an automatic transmission for a motor vehicle.
2. Description of the Related Art
As this kind of control apparatus, there is known the following in Japanese Published Unexamined Patent Application No. 168471/1991. Namely, a pair of first and second pressure regulating valves are provided. The first pressure regulating valve is used exclusively for supplying hydraulic oil and the second pressure regulating valve is used exclusively for discharging hydraulic oil. Pressure rise (or pressure boosting) characteristics of a hydraulic engaging element on an engaging side to be engaged in carrying out speed changing are controlled by the first pressure regulating valve. Pressure drop (or pressure decrease) characteristics of a hydraulic engaging element on a disengaging side to be disengaged in carrying out speed changing are controlled by the second pressure regulating valve. Thus, there is an attempt to improve driver "feel" at the time of speed changing.
The pressure regulating valves of the above-described conventional control apparatus are, generally speaking, electronically-controlled with solenoid valves. Therefore, the regulating valves sometimes do not function normally due to a failure in an electronic control system.
In case the second pressure regulating valve does not function normally with the result that a hydraulic pressure (or a hydraulic oil pressure) of the hydraulic engaging element on the disengaging side is maintained at a high pressure, there will occur locking of the transmission due to simultaneous engagement of unintended gears. As a solution, in the above-described conventional control apparatus, the hydraulic oil is discharged from the hydraulic engaging element on the disengaging side without passing through the second pressure regulating valve at a lapse of a predetermined time from the start of speed changing, whereby the hydraulic pressure in the above-described hydraulic engaging element is forcibly decreased.
However, in the above-described conventional control apparatus, the hydraulic oil is supplied to a hydraulic engaging element on the engaging side through the first pressure regulating valve not only at the time of speed changing but also after the speed changing has been completed. Therefore, in case the hydraulic pressure in the hydraulic engaging element on the engaging side cannot be increased or boosted due to a functional abnormality in the first pressure regulating valve, the hydraulic pressure in the hydraulic engaging element on the engaging side cannot be boosted or increased and, therefore, the transmission will revert to a neutral condition.
Further, in the above-described conventional control apparatus, the hydraulic pressure in the hydraulic engaging element on the engaging side must be boosted by the first pressure regulating valve up to a line pressure. Therefore, the range of hydraulic pressure to be controlled becomes wider, and a delicate control of the hydraulic pressure in a transient region of engagement at a relatively low pressure becomes difficult.
In view of the above-described disadvantages, the present invention has an object of providing a control apparatus for a hydraulically operated vehicular transmission in which the transmission will not revert to a neutral condition even when a functional abnormality occurs in the pressure regulating valves, in which locking of the transmission due to simultaneous engagement of unintended gears can be prevented, and in which a delicate control of the hydraulic pressure in a transient region of engagement can be provided with a higher accuracy.